Abstract
Solar ultraviolet (UV) irradiance is a key driver of climatic and biotic change. Ultraviolet irradiance modulates stratospheric warming and ozone production, and influences the biosphere from ecosystem-level processes through to the largest scale patterns of diversification and extinction. Yet our understanding of ultraviolet irradiance is limited because no method has been validated to reconstruct its flux over timescales relevant to climatic or biotic processes. Here, we show that a recently developed proxy for ultraviolet irradiance based on spore and pollen chemistry can be used over long (105 years) timescales. Firstly we demonstrate that spatial variations in spore and pollen chemistry correlate with known latitudinal solar irradiance gradients. Using this relationship we provide a reconstruction of past changes in solar irradiance based on the pollen record from Lake Bosumtwi in Ghana. As anticipated, variations in the chemistry of grass pollen from the Lake Bosumtwi record show a link to multiple orbital precessional cycles (19–21 thousand years). By providing a unique, local proxy for broad spectrum solar irradiance, the chemical analysis of spores and pollen offers unprecedented opportunities to decouple solar variability, climate and vegetation change through geologic time and a new proxy with which to probe the Earth system.
Highlights
Earth’s energy budget is governed by incoming solar irradiance[1]
There is considerable uncertainty with regard to how UV flux varies through time[4], but over longer timescales orbitally-modulated variations in total solar irradiance (TSI) should be the major control on UV flux[24], and UV absorbing compounds (UACs) concentrations should track modelled TSI
Lycopodium is an ideal taxon for assessing latitudinal trends in UAC concentrations because (a) it is geographically and latitudinally widespread; (b) its chemistry is well-understood from previous research on UV perception and reconstruction[13,15,17,18,21]; and (c) the morphology of the plants, with sporangia facing upwards, means that the developing spores receive a clear UV signal
Summary
Concentrations; a multiple regression model with both modelled TSI and Poaceae proportion as a proxy for habitat openness gives a significant positive relationship (Fig. 3a) (see Methods). The association between UACs and TSI holds when age-depth model uncertainty is taken into account (Fig. 4 and Supplementary Fig. S2) These pollen samples have been processed as standard palynological preparations (see Methods), the potentially damaging oxidation step has not removed or obscured the UAC signal[21] (Supplementary Fig. S3). Tree-covered savannahs shading levels will vary across the landscape, leading to a more heterogeneous UV environment than open grasslands (represented by high Poaceae proportions) or closed forests (low Poaceae proportions), and an higher within-sample variability of UAC concentrations Consistent with this expectation, there is a humped relationship between the within-sample UAC standard deviation and the proportion of grass in the pollen sum (Fig. 3c) (see Methods). The UAC-based approach outlined here can be used to reconstruct UV and TSI from the extensive sporomorph archives that are present in the sediment and rock records, and available ready-processed in palynological laboratories around the world
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